When I've designed and saved my for when I get someone to enter the data
they can't save it as a completed for, how do I enable this function?
If the user has Acrobat Pro, they can save the data. If they only have Reader you need a product called Adobe Reader Extensions that enables save rights (if the form was created in Designer 7). It is an addition you have to purchase for big $$$. The newer versions of Acrobat Pro 8+ have limited save capabilities that can be added to the form.